1. Field of the Invention
Methods and devices consistent with the present invention relate to reproducing encrypted content and approving reproduction of the encrypted content, and more particularly, to decrypting and reproducing encrypted content by using a security mediated module and a reproduction device to which a first partial key and a second partial key, for constructing a device key, are respectively allocated.
2. Description of the Related Art
Recently, various software titles (hereinafter referred to as “content”) such as a game program, voice data, image data, a Word document preparation program, and the like are distributed via a network, such as the Internet, or storage mediums, such as a Digital Video Disc (DVD), a Compact Disc (CD), or the like. Such distributed content may be stored in a storage (recording) medium, such as a memory card, a hard disk, or the like, wherein the storage medium is installed in a reproduction device, such as a Personal Computer (PC), a game apparatus, or the like, which is owned by users. After the content is stored, the stored content may be used by reproducing the content from the storage medium.
In general, for software content, such as a game program, voice data, image data, and the like, distribution rights that are a kind of copyright are granted to manufacturers and content providers (CPs) related to the software content. Thus, in distribution of the software content, a certain usage prohibition is generally set so as to allow only a formal user to use the software content, and security is considered so that unauthorized copying of the software content is not performed.
One of the methods of prohibiting usage by users is to encrypt content that is to be distributed, and to make only an authorized user obtain a means by which the content is to be decrypted. For example, when various kinds of encrypted content, such as voice data, image data, a game program, or the like are distributed via the Internet, or the like, a content key that is a means for decrypting the distributed encrypted content is also given to a user that has been confirmed to be an authorized user.
As illegal copying of content in multimedia contents industry has recently increased, a variety of technologies for protecting content have been researched and developed. Such examples include a Conditional Access System (CAS) for protecting broadcasting contents, Digital Rights Management (DRM) for determining whether to allow digital content to be used according to rights regarding the digital content, and digital content protection technologies for storage media such as a Content Scrambling System (CSS), Content Protection for Recordable Media (CPRM), and an Advanced Access Content System (AACS).
These technologies for protecting content usually encrypt the content by employing encryption technologies and the like, so that the contents can be distributed or sold to users, and the contents can be protected by controlling only an authorized user access to a content key by which the contents can be deciphered.
Here, a related art method of encrypting data encrypts data by using a public key or a secret key of another party, and transmits the data. However, in the case of multimedia data having a huge amount of data, separate encryption of content by each user has very low efficiency since high-volume content is encrypted with keys that are always different. Thus, in most cases, content protection systems use a two-step encryption method in which one content key is used to encrypt the same contents. However, in some cases, user keys respectively owned by each user are used to encrypt the one content key.
The user key may be a device key stored in various devices using content, such as a digital television, a set-top box, an MPEG Audio Layer-3 (MP3) player, a portable video player, a DVD player, a Blu-ray player, and the like.
Many content protection systems encrypt a content key by using a key of a content device, thereby controlling content to be used only in a specific content device. That is, content is encrypted with a content key and distributed. Also, the content key is encrypted with a device key and distributed. Thus, a content device receives the encrypted content and content key, decrypts the encrypted content key by using the device key, and then finally decrypts the encrypted content by using the decrypted content key.
A content key has to be distributed to only an authorized user. Thus, when a distributed reproduction device is revoked by being illegally copied, the reproduction device should not be considered any more as an authorized user.
FIG. 1 is a diagram illustrating a related art process of revoking a device when a key of the device has been leaked.
Referring to FIG. 1, an case is illustrated in which a device key is illegally copied and leaked among devices which are authorized to use a storage medium (title 1) 110 having recorded thereon predetermined encrypted software content, when the storage medium (title 1) 110 is being distributed.
Regarding a device 120, a key of which is leaked, and a clone device 130, a CP may later obtain information on the leaked device key via a key tracing algorithm 140.
After that, the CP revokes the corresponding devices 120 and 130, and then encrypts corresponding software content by using a new key.
Thus, title 2 160 of the newly encrypted software cannot be reproduced in the device 120, the key of which is leaked, or in the clone device 130.
A related art revocation system is performed by using the aforementioned method. However, such a method has problems since the method can only protect content, e.g., the title 2 that is newly encrypted and distributed after an illegally copied device is revoked, but cannot retroactively prevent content, e.g., the title 2 that is previously encrypted and distributed, from being reproduced.